Hey Fellas,

In the last dozen time I've driven my car I've gotten 3 separate episodes of codes. First two times I got only a code P0171 (Lean Bank 1). Last time I got both P0171 and P0174 (Lean Bank 2). Here's the weird thing: the car runs just fine. In fact, the codes are only set if the car is warm and I've been sitting at a light for at least 30 sec (typically more like 1 minute). In fact I took a trip of about 175 miles and didn't get a check engine light the whole way (mostly highway driving). I haven't even noticed a decrease in fuel economy. I don't get the codes set every time I drive.

Since I'm getting lean indication on both banks, I'm thinking it's not an exhaust leak. More likely it's something upstream. I'm guessing a dirty MAF or an intake leak? Any other ideas before I start tearing the car apart?

Thanks in advance.

 

Needs new O2 sensors....

 

New O2 sensors are a possiblity, but what are the odds that at least 2 of the 4 sensors die within a week of each other on different banks? When it was just one side indicating lean that was my first guess. But now that both sides are coming up lean, I'm thinking it's something upstream of the exhaust that would affect both banks.

 

If you have a lean condition on both sides, it may be a problem with something like the fuel pressure or the MAF. The codes set when the long term fuel corrections approach their upper limits (+25%). That could mean that the long terms are close to maxing out, but are still able to pull the A/F ratio back where it belongs. Could be running normally now, but eventually, you'll reach the limits of the long term fuel corrections to add sufficient fuel to make up for the lean condition.

Low fuel pressure, dirt on the MAF sensor, a leak in the elbow between the MAF and the TB, a large vacuum leak, etc could all cause the problem. Put a scanner on it and check the long term fuel corrections. Since it seems to happen primarily at idle, if its mainly the elevated Cell 16 corrections that are contributing to the problem, I would suspect a smaller vacuum leak.

 

Typically old O2 sensors cause your engine to run rich, not lean.

 

I had exactly the same condition, P0171 & P0174 codes with an engine that ran perfectly, except for a higher than normal idle. The most common cause of these codes, as was my problem was a vacuum leak. Check ALL your vacuum hoses....I'm sure you'll find a leak somewhere.

 

Thanks Fellas it was a dirty MAF. 3 Q-tips later all is better - cheapest fix i've ever done. you do have to VERY careful with those filaments.